Fuel cells covert the chemical energy found in a fuel (e.g., hydrogen) into electricity through a chemical reaction with an oxidizer (e.g., oxygen). One example of a fuel cell is a Solid Oxide Fuel Cell (SOFC). Typical SOFCs operate between about 500-1000 degrees Celsius, and the conversion process is exothermic. SOFCs include an anode, a cathode, and an electrolyte between the anode and cathode. The electrolyte may be a solid oxide or a ceramic electrolyte. Hydrogen is introduced to the SOFC at the anode, where the hydrogen atoms are stripped of their electron by a catalyst to produce hydrogen ions. Oxygen is introduced to the SOFC at the cathode, where the oxygen is reduced into oxygen ions. The electrolyte between the anode and the cathode conducts the oxygen ions from the cathode to the anode, where the oxygen ions oxidize the hydrogen ions. In this reaction, a water byproduct is given off along with electrons. The water is typically vented from the SOFC as waste. The electrons produce an electrical current output from the anode, and the electrons return to the cathode to complete the electrical circuit and start this electrochemical process over again.
SOFCs can operate either forward or backward. In the forward mode or fuel cell mode, hydrogen and oxygen supplied to the SOFC is converted into water vapor via oxidation to generate electricity. In the backward mode or electrolysis mode, electricity and water supplied to the SOFC is converted into hydrogen and oxygen via electrolysis. Hydrogen generated in the electrolysis mode is reclaimed for use as a fuel when the SOFC is operated in the fuel cell mode. The oxygen is typically vented as waste. SOFCs that operate in both a fuel cell mode and an electrolysis mode are referred to a Reversible SOFCs (RSOFCs).
Due to their bi-directional conversion abilities, RSOFC systems are being investigated for use in powering and storing energy in grid systems that utilize inconsistent power generation, such as solar and wind. In these environments, RSOFCs can utilize the excess power supplied by solar or wind while operating in electrolysis mode to electrolyze water for storage as hydrogen. During periods where additional power is desired by or when power is not being generated by solar cells or wind turbines, the RSOFC is operated in fuel cell mode to generate electricity using the stored hydrogen.
In electrolysis mode, the RSOFCs utilize a water source and electricity to convert water into hydrogen. Typically, the water source is a municipal water source, which is filtered and de-ionized prior to being used by the RSOFC. In some cases, sea water can be desalinated and de-ionized for use by the RSOFC if a municipal water source is unavailable. Also, a combination of both can be used depending on the reliability of the municipal water source.
Although municipal water sources and/or sea water may be available, there is still a desire to minimize the water consumption of a RSOFC system to reduce its reliance on a source of water. This goal is referred to as a water neutral balance for a RSOFC system.